Human cytomegalovirus (HCMV) is a widespread beta-herpesvirus. Although HCMV infection is usually asymptomatic in healthy children and adults, it is a significant cause of morbidity and mortality in newborn and immunocompromised individuals. A possible connection between HCMV and human cancer has been investigated for many decades, and numerous associations between HCMV and human malignancies have been reported. HCMV DNA and/or gene products have been found in a wide variety of tumor samples; and there is extensive overlap between HCMV biology and the hallmarks of cancer, including the ability of HCMV to sustain cell proliferation and migration, induce angiogenesis, induce genomic instability, manipulate the DNA-damage response and promote inflammation while avoiding immune detection. However, in spite of these associations, there is no compelling evidence that HCMV is a tumor virus, i.e., a virus that can contribute directly to the initiation and maintenance of a cancer. An association without proof of causation has led to the concept that HCMV is an oncomodulatory agent, i.e., it has the potential to modulate the malignant properties of tumor cells although it doesn?t directly induce the oncogenic transformation of cells. This proposal builds on our recent observation that HCMV infection induces a phenotype associated with both normal development and cancer, a mesenchymal-to-epithelial transition (MET). Infection induces this phenotype in ?normal? diploid cells, which host active viral replication and are killed by HCMV, and in mesenchymal breast and glioblastoma tumor cells, which survive infection. Since it is possible that HCMV might infect a tumor cell, induce a change and then be lost from the progeny of the altered cell, the culture system will identify tumor cells that have been infected by HCMV, irrespective of the continued presence of the viral genome. Lox sequences in tumor cells will be rearranged by virus expressing Cre recombinase to activate marker genes, marking infected cells and their progeny. The system will report when a cell has been infected, and provide the opportunity to delineate the full range of alterations induced in that cell and its progeny, initially via RNA-seq analysis. Thus, this proposal is designed to answer a long-standing question in HCMV biology: can HCMV infection induce inherited changes in tumor cells? In the longer term, if alterations are identified, they will generate hypotheses that can be tested for how HCMV might alter the behavior of tumor cells.